On the ServerGrove blog there's a recent post about the speed boost that you can get using APC caching (with an example of a symfony application's perfomance).

I've been developing a website based on symfony. Since symfony 1.3 and 1.4 came out this week, I decided to upgrade it to 1.3 and then when I feel confortable to 1.4. [...] Then, I decided to run some benchmarks using ab (Apache Benchmark) to see how symfony 1.2 and 1.3 compared. To my surprise, I did not see an improvement on speed. I was expecting the new version would be faster, but for some reason I am not seeing any improvements.

When his testing showed no noticeable results, he turned on view caching in the framework. This helped a good bit but the real boost came when he implemented APC caching. His requests per second jumped from fourteen up to seventy, a five-fold improvement.